Osteoporosis afflicts 25 million women and men in the US per year and burdens the health care system with over 20 billion dollars per year. As the US population ages and lives longer, development of prevention strategies and new treatments for bone loss become even more crucial. The Principal Investigator identified a gene, Lrp5, responsible for high bone mass in a human kindred and evidence now exists that this molecule is involved in bone mass regulation through its role in the mechanotransduction pathway. In this proposal, we have developed a unifying hypothesis and model that integrates PGE2 and the Wnt/(-catenin signaling pathways and accounts for the role of Lrp5 in regulating bone cell function in response to loading. We will test the hypothesis that crosstalk between the prostaglandin and Wnt/( catenin signaling pathways in osteocytes is a key early event in the bone response to mechanical loading that sets into play an amplification mechanism to propagate the signal to load, resulting in a bone formation response. We propose that loading is first perceived by osteocytes followed by signal propagation to lining cells and osteoblasts on the bone surface. It is known that an early osteocyte response to mechanical loading is PGE2 release which functions in an autocrine/paracrine fashion to induce new bone formation. We propose this early event results in cross-talk with and activation of the Wnt/(-catenin pathway in a first step that is independent of Lrp5 receptor activation. This initial activation alters the expression of key factors that initiate the activation of Lrp5 through a paracrine/autocrine feedback mechanism, generating a secondary activation (amplification) of the Wnt/(-catenin pathway at the level of Lrp5. To test this hypothesis the following Specific Aims are proposed: 1) Determine when and which cell types (osteocytes, lining cells and osteoblasts) activate the Wnt/(-catenin signaling pathway in the adult skeleton in response to changes in mechanical load. 2) Determine the temporal relationship and crosstalk between the PGE2 and the Wnt/(-catenin signaling pathways in the bone cell response to mechanical loading. 3) Determine the mechanism by which the expression of known modulators of Lrp5 (Wnts, Dkks, Wise, Sost) are altered by mechanical loading in adult bone cells. These specific aims will be accomplished using complementary in vitro, in vivo and transgenic approaches to dissect the molecular mechanism for PGE2 initiated activation of (-catenin signaling and the role of Lrp5 in the response of bone to mechanical loading. These studies will provide a unifying model for integrating two pathways known to be important for bone responsiveness to mechanical loading, prostaglandin and Wnt/(-catenin signaling, and will result in a conceptual framework to integrate other important signaling pathways in bone (ATP, Ca2+ fluxes, integrin, etc.) into a paradigm that ultimately explains how bone responds to mechanical loading at the cellular and molecular level. This will lead to new approaches to treating and/or preventing osteoporosis.